A repository of three-dimensional structural information about nucleic acids

Wiring diagrams of molecular interactions, reactions, and relations

Enregistre les messages de Twitter...

Introduction to Molecular Biology Word

Molecular biology is the scientific discipline that seeks to fully understand the molecular basis of heredity, genetic variation, and the expression patterns of individual units of heredity
called genes. Molecular biologists study how biological characteristics are passed on from one generation to the next and investigate how bacteria or viruses function at the molecular
level.

Molecular biologists identify living things called micro-organisms that are too small to be seen without a microscope to better understand their relation to human,
plant, and animal health. They look at these organisms in relation to genetics in order to understand the inheritance and molecular basis of genetic disease. They also investigate how these
micro-organisms function in the production of vitamins, antibiotics, amino acids, alcohols, and sugars. The results of their research has produced breakthroughs in medical, agricultural,
industrial, and other scientific fields.

Many molecular biologists believe that a chain of molecules connects each of us with the first living organisms that arose on the Earth billions of years ago. Therefore, people are all connected
biologically in some form or another. Yet interestingly, all humans have individual characteristics that distinguish them from one another. Molecular biologists conduct research into the
biochemical and physiological aspects of heredity, particularly the role of deoxyribonucleic acid (DNA), and carry out molecular tests to aid in diagnosing disease.

Molecular biologists help scientists and physicians in the understanding, diagnosis, prevention and treatment of infections in animals and humans by investigating how organisms cause disease and
their role in disease processes. Work in molecular biology is often interdisciplinary, so they may work closely with chemists, biochemists, geneticists, genetic engineers, pathologists and other
physicians, environmentalists, civil engineers, veterinarians and geologists.

Molecular biologists use a variety of specialized sophisticated equipment such as gas chromatographs and high pressure liquid chromatographs, electrophoresis units, thermocyclers, fluorescence
activated cell sorters and phosphoimagers. They may also use computers in conducting experiments. It is common to find a molecular biologist peering through the lens of a microscope or performing
other related experiments in a laboratory. However, the nature of the work may vary considerably with each assignment.

DNA

DNA (deoxyribonucleic acid) is the information storage molecule for the cell. Each cell contains within
the DNA the entire instruction set to produce and run an entire organism (including you). In multicellular organisms, most of the DNA is stored in a
central location within the cell, called the nucleus.

Bases

DNA is composed of only four bases, which can be thought of as letters of the DNAalphabet. The structure of these four letters is shown in
the figure to the right. There are
two purinebases (adenine and guanine) and
two pyrimidinebases (thymine andcytosine). The bases are commonly abbreviated with the first letter
of their names:

Nucleotide Bases

Base

Abbrev.

adenine

A

guanine

G

thymine

T

cytosine

C

Nucleotides

When a base, such as adenine,is linked to a deoxyribose sugar through a glycosidic bond, the structure is called a nucleoside. When the deoxyribose sugar
isphosphorylated, on either the 3' or the 5' position (or both), the structure is a called a nucleotide. The precursors
of DNA synthesis are deoxynucleoside-5'-triphosphates or dNTPs.

DNA structure

DNA is composed of a polynucleotide (multiple nucleotides) chain that is formed by linking nucleotides through 3',5'- phosphodiester bonds. In this
way, DNAforms a long chain of sequential nucleotides.

DNA chains are not usually just single strands
of nucleotides.

Usually, nucleotides of two polynucleotide strands are base-paired to each other. The nucleotide base adenine pairs through hydrogen bonds with
the nucleotide thymine forming an adenine-thymine (A-T) base pair. Guaninebinds with cytosine forming a guanine-cytosine (G-C) base
pair. Because of this base pairing, polynucleotide chains in double-stranded DNA are always complementary to each other.

DNA Double Helix

The nature of the bonds between nucleotidebases results in the famous "double helix" DNA structure. The hydrogen bonding between
strands of DNA holds the strands together until they are separated for DNA replication or RNAtranscription.

DNA Replication

DNA is replicated (reproduced) through an extremely complicated process. This is the simplified version. The two strands of DNAare separated and a new complementary strand is synthesized for each old strand
by adding complementary base pairs to the newly-synthesized strand (see figure at right). The DNA polymerase (the enzyme that makes
new DNA) reads the base on the original strand and adds the complementary base to the newly synthesized
strand. DNA is always synthesized in a 5' to 3' direction. Since one strand runs in the opposite direction, it must be synthesized in pieces that are later connected
with DNA ligase. In reality, both strands are synthesized in pieces, since the DNA is unwound at numerous sites simultaneously on
each chromosome in order to produce a complete copy in a short period of time (hours instead of weeks).

RNA

How does DNA turn in you ?

Since DNA just stores information, there needs to be a way to turn the information into physical bodies. This feat is accomplished through the synthesis of an
intermediate information-carrying molecule that carries the information from the nucleus to the cytoplasm, where it is processed
into proteins. The intermediate molecule is called RNA (ribonucleic acid). As you can tell from
the name, it differs from DNA in that part of it (the ribosesugar) has an extra oxygen compared to DNA (the "deoxy" version). In
addition, RNA does not use thymine as a base, but another nucleotideknown as uracil (see
figure right). Uracil forms base pairs with adenine, as thymine does
in DNA. RNA is transcribed from DNA through an enzyme called RNA polymerase, in a process
that is known as transcription. There are several kinds of RNA, which have different functions. However, the RNA that eventually codes
for proteinsis called messenger RNA (mRNA).

Translation

Once mRNA is synthesized,
it is processed (usually through splicing) in the nucleus before moving to the cytoplasm. In many instances, alternate splicing of
an RNAtranscript can produce multiple proteins. It is one of the chief ways that 25,000 genes in the
human genome can make ~100,000 proteins. Proteins are produced from an mRNA template using a process known
as translation. The translation process is quite complicated and requires numerous molecular machines and co-factors. This description is a
simplified version of what happens. A large molecular machine, called the ribosome, binds to the mRNA. Ribosomes are composed
of two subunits. The large subunit is composed of three forms of ribosomalRNA (rRNA) and 49 different proteins.
The smaller subunit is composed of one rRNA and 33 different proteins. A third kind of RNA,
transfer RNA (tRNA), carries amino acids that are used to make proteins according to
the mRNAtemplate. One end of tRNA, consisting of three bases, called the anticodon, binds to a series of three
complementary bases, called the codon on the mRNA strand. The other end of the tRNAbinds to a
specific amino acid. Specific sequence on the RNA are translated into proteinsaccording to
the genetic code. Each set of three nucleotidebases codes for one amino acid. This code is nearly universal
for all organisms (with a few exceptions). The code is reproduced below:

The Genetic Code

The 30s subunit of the ribosome binds to the mRNA. A specific tRNA (usually the
first codon codes for methionine) binds to the codon of
the mRNA The 50s ribosomal subunit binds to the 30s subunit. The next tRNA binds to the
next codon on the mRNA and numerous initiation factors and the enzyme peptidyl
transferase link the two amino acids together through a peptide bond. As
the ribosomal complex moves along the mRNA, the peptide chain gets longer until the
entire protein is formed. Other molecular machines are responsible for making sure that the protein folds into the correct
three dimensional shape.

followed by:

Ribosomes

Ribosomes are small organelles found in the cells of all life forms. They are quite small, only a few hundred nanometers across. It has been theorized that ribosomes were once independent life
forms, reproducing on their own, until other chemicals such as carbohydrates and DNA were brought into the fold at a later time. Ribosomes are composed of rRNA (ribosomal RNA, or ribonucleic
acid) and other catalytic proteins. Their main function is to produce a variety of proteins from simple genetic instructions which propagate outwards from the cellular nucleolus in the form of
messenger RNA (mRNA). Ribosomes float in the cytoplasm of a cell or bind to the endoplasmic reticulum, ribbon-like structures found within the cell.

Ribosomes are sometimes referred to as simply RNA. Like DNA, they are long chains of amino acids, but their base pairs are different and they are usually not as long as DNA. Ribosomes play a key
part in protein synthesis, the process that generates organic tissue. Genetic instructions for the creation of new proteins come from mRNA. Ribosomes always have two subunits which interlock and
behave as a single entity.

The exact type of ribosome found within a cell can vary based on the kind of organism that the cell is a part of. Eukaryotes (multi-celled organisms) have their own kind of ribosome, whereas
prokaryotes (single-celled organisms) have another. Certain organelles within the cell, chloroplast and mitochondria, have their own distinct ribosomes as well. Ribosomes make up the majority of
RNA content within a cell, being responsible for about 95%.